System powered actuating means for butterfly and other dampers find increasing favor in air conditioning systems. Further, such actuating means have heretofor been designed to obviate the need for spring loaded butterfly dampers by employing velocity pressure derived actuating forces for closing the dampers. Such a damper is illustrated and described in U.S. Pat. No. 3,361,157 to Schach and actuating means of this type have been generally satisfactory. Certain disadvantages are encountered, however, in the use of velocity pressure to create a damper closing force. More particularly, it is necessary to limit the range of movement of the damper so that the damper is never permitted to completely close or completely open. Obviously, a damper which is not fully open creates an unnecessary and undesirable pressure drop at high flow conditions and, the inability to fully close the damper of course results in system leakage.